An older brother notices his younger sister Aoi's suspicious behavior. When he questions her, she says she was practicing putting on a condom with her brother's penis. Since then, he has been helping Aoi practice putting on a condom, but it's too painful to leave him erect, so he asks Aoi for help with ejaculation. He can't endure just a handjob, so he gives her a blowjob and a pussyjob... Finally, the two of them get close to penetration, and they decide on a rule that "if the tip is only 3cm deep, it doesn't count as sex," and they start having sex with each other. Before long, just inserting the head of the penis isn't enough, and they start wanting to penetrate each other all the way to the back of the vagina.